Release papers are used e.g. as backing papers for adhesive labels. The base paper is coated with silicone usually with a solvent-free silicone system. As this method has become more common, the surface properties of the base paper for the release paper have become more significant in achieving a good siliconized surface quality. Due to the properties of silicone, the low porosity, low water and oil absorptivity, and high smoothness of the base paper have become more important.
The functional demands on the base paper for release paper are complex, due to a wide range of usage and difficult applications. The essential properties of base paper include: strength properties (also after thermal treatment), dimensional stability, high density, barrier properties, chemical suitability for siliconization, high transparency, low porosity, and good smoothness. A well-functioning base paper is passed without breaks on a converting machine, requires little silicone for good silicone coverage, functions without problems in die cutting of a laminate, and does not cause problems in labelling.
To improve the quality of the base paper, it is known to surface size the surface, which receives the release agent, with a suitable substance and to calender it after the surface sizing. The quantity of surface size in the base paper is typically at a level of approximately 3 g/m2/side, calculated as the amount of dry matter. The surface size agents are, however, relatively expensive, and therefore it would be beneficial to decrease the quantity of the surface size. Even if a substantial quantity of surface size were used, there is the possibility that there are pores in the base paper, which prevent a good silicone coverage, in which case the adhesive penetrates the pores, and the releasability of the label from the release paper declines.